In the field of vehicle propulsion, where drive torque is transferred from an engine through a transmission to the vehicle driven wheels, it has become generally known that a hydrodynamic device can be employed. Such a device is generally positioned between the engine and the transmission, and "softens" the transient spikes of high torque or high speed engine shaft movement, preventing an unpleasant sensation from being translated to the driver. If the hydrodynamic device is a torque converter, then the engine output torque can also be multiplied to assist in accelerating the vehicle from a standing start, or from a low speed to a higher speed.
When fluid couplings and torque converters were employed, it also became known to provide a lockup clutch for mechanically locking the input member of the hydrodynamic device to the output member. By controlling the clutch to effect such lockup after the vehicle has attained a predetermined, low speed such as 10 miles per hour (mph), considerable savings can be realized by obviating the spin losses in the coupling itself. This technology was given fresh importance in 1973, when the price of crude oil began to climb rapidly. Different types of lockup clutches were employed, but there still remain several shortcomings of such clutches.
One such disadvantage of known devices is that the output shaft speed was generally sensed by taking a signal from the governor valve fluid line. For some purposes this may be adequate but it does not provide a convenient, accurate reference for regulating the precise point of clutch lockup, and the exact time of clutch release. In addition, it has been difficult to obtain precise control in the region encompassing the lockup and release points of the clutch. It is thus a primary consideration of the present invention to produce an improved control system which achieves more effective control and does not utilize the governor line as a speed-indicating signal.
When the transmission used with a fluid coupling or torque converter is a continuously variable transmission (CVT), this leads to an additional difficulty. When the adjustable pulleys are in the overdrive condition and the vehicle is moving at a normal speed, if the brake pedal is sharply depressed in a "panic stop" mode, the vehicle may be rapidly halted before the CVT pulleys are shifted to the overdrive position, to obtain maximum torque at the next start-up. If a metal belt is used with metal pulleys in a CVT, this can result in damage to the belt as the CVT is forced into the underdrive position after the vehicle has stopped. It is thus another important consideration of the present invention to provide a control system which virtually assures the CVT will be returned to the overdrive ratio position before the vehicle is halted, even in a panic stop condition.